Grid tightness multiplier (multiplies resolution) for discarding values far from a sample.
Default of zero discards nothing. For Lidar bin gridding you typically do NOT want to use 0 as it
it can be very slow to fill all gaps in irregular Lidar data. Use GM_GRID_DISABLE_GAP_FILL to disable
gap filling altogether. For Lidar bin grids, gap fill values in the range 1-32 are common.